The invention relates to a method of manufacturing a magnetic field sensor or magnetic memory having at least two magnetoresistive bridge elements in a (half)Wheatstone bridge arrangement, each magnetoresistive element comprising a free and a pinned ferromagnetic layer, said at least two bridge elements having pinned ferromagnetic layers with different magnetization directions.
Such a method is known from U.S. Pat. No. 5,561,368.
U.S. Pat. No. 5,561,368 describes a method of manufacturing a magnetic sensor having a full Wheatstone bridge arrangement in which each adjacent resistor element has an equal but opposite response to an applied magnetic field. Each sensor element comprises a pinned and a free ferromagnetic layer, and pinned ferromagnetic layers of adjacent elements have opposite relative magnetization directions. The magnetic direction of the pinned ferromagnetic layers is pinned by means of a conductive fixing layer which is provided electrically insulated above the resistor elements. A current through the conductive fixing layer generates a magnetic field which pins the magnetic direction of the pinned ferromagnetic layers.
Many sensor applications require the use of a Wheatstone-bridge configuration, to eliminate the unwanted resistance change due to temperature variations. Whereas for sensors based on the Anisotropic Magnetoresistance (AMR) effect a Wheatstone-bridge can be elegantly realized using barber-pole structures, for Giant Magnetoresistance (GMR) and Tunneling Magnetoresistance (TMR) this is not simple at all. So far the following possibilities have been proposed:
1. Two (of the four) bridge elements are magnetically shielded; the shields may be used as flux concentrators for the two sensitive elements. However, only two of the bridge elements are then effectively used, which reduces the output signal.
2. An insulated conductor is integrated below or over the sensor elements (consisting of exchange-biased spin valves) to induce a magnetic field that xe2x80x98setsxe2x80x99 the exchange-biasing direction of the elements in opposite directions, while the elements are heated above the blocking temperature of the exchange-biasing material. A comparable method with an integrated conductor has been proposed for elements based on an Artificial Antiferromagnet (AAF). This method is used in U.S. Pat. No. 5,561,368.
3. In the factory the magnetizations are set in opposite directions in different branches of the bridges by exposing the wafer with sensor structures to an external magnetic field that is induced by a kind of xe2x80x98stampxe2x80x99 comprising a pattern of current carrying conductors which is brought in the vicinity of the wafer. This method is in some sense equivalent to the method described sub 2.
All these possibilities are rather complicated and require quite some effort in practice. Moreover, the possibility mentioned sub 1 only allows the realization of a half-bridge and therefore loses half of the possible output signal. The magnetic fields that can be realized with the options 2 and 3 are very limited in strength, because the currents have to be relatively small in the (necessarily narrow and thin) conductors. Furthers, option 1 and 2 require several extra processing steps (both for patterning and insulation of the conductors or shields), which makes the sensors more expensive and reduces the manufacture yield. If option 3 is used, the sensor may be destroyed if exposed to a magnetic field of the same strength (or larger) as the field used during setting of the magnetization directions. In this case (after packaging) it is almost impossible to reset the sensor, certainly without the specific magnetization device.
The robustness of the sensor elements becomes more and more important, in particular for automotive applications, but also for read heads. This trend makes setting of the magnetization directions after deposition of the elements more and more difficult. For example, GMR material very suitable for automotive sensor applications shows no rotation of the exchange-biasing direction when exposed to a magnetic field of approximately 50 kA/m at temperatures up to 450xc2x0 C. It is difficult to generate fields of such strength by the above method 2 or 3. An intentional reduction of the robustness of the material, however, would degrade the characteristics of the sensor, which poses a problem in particular for automotive applications.
It is an object of the invention to provide a manufacturing method as given in the opening paragraph which resolves at least partially or preferably eliminates some or all of the above-mentioned problems.
To this end, the method in accordance with the invention is characterized in that in a first deposition step a first ferromagnetic layer of one of the two said elements is deposited, during which deposition a magnetic field is applied to pin the magnetization direction in the first ferromagnetic layer in a first direction, after which in a second deposition step a second ferromagnetic layer of the other of the two said elements is deposited, during which deposition a magnetic field is applied to pin the magnetization direction in the second ferromagnetic layer in a second direction different from, preferably opposite to the magnetization direction in the first ferromagnetic layer.
In the method of the invention the at least two pinned ferromagnetic layers are fabricated in at least two separate deposition steps, and during the deposition steps magnetic fields are generated by which opposite magnetic directions are imparted to the said pinned ferromagnetic layers. Preferably this is achieved by using magnetic fields of opposing directions during the first and second deposition step. This method is simpler than methods in which magnetic fields are used with the same direction but in which the position of the device is changed.
Although the method is useful for devices having elements with anisotropic magnetoresistance, it is of particular usefulness for devices which use the Giant MagnetoResistance effect (GMR) or Tunneling MagnetoResistance effect (TMR).
Preferably, the magnetic field applied during the second deposition has a direction different from, preferably opposite to, the direction of the field applied during the first deposition, while the position of the device during deposition is the same. Alternatively, but less favored, the magnetic field applied during deposition is the same, but the position of the device is changed between depositions to achieve the same result. Although the method is applicable to the manufacture of a device having a half-Wheatstone bridge arrangement, it is of particular importance for a device having four bridge elements in a Wheatstone bridge arrangement.
A Wheatstone bridge arrangement comprises at least four bridge elements in a bridge arrangement.
A further embodiment of the invention is characterized in that two Wheatstone bridge elements are manufactured, the magnetic directions of the corresponding elements enclosing an angle with each other, preferably of 90xc2x0.